The shopping cart will send your shoppers email receipts. Some email servers will not accept this mail unless you have authorized our mail systems to send mail on your behalf. To do this you need to contact your IT staff and have them edit the DNS records for your domain.

They need to add an SPF record. Like the one shown here:
v=spf1 a:smtp.cartmanager.net

Remember to add your own IP address/Mail sender as well as the one shown here.